1. Field of the Invention
The present invention relates to a conveying method and a conveyor control device and more particularly to a conveying method and conveyor control device used in a conveying system that transfers workpieces such as lead frames, circuit boards, etc. in, for example, a semiconductor assembly line.
2. Prior Art
The Japanese Patent Application Laid-Open (Kokai) No. 3-158317 discloses a conveyor system used in a semiconductor assembly line. In this system, a plurality of processing devices of the same type (that are all wire bonders) are installed, and workpieces supplied to each one of the processing devices and the workpieces discharged from each one of the processing devices are conveyed by a single conveyor line that is commonly used to those processing devices.
In this conveyor system, as shown in FIG. 5, pairs of conveyors 15A.sub.1 and 15A.sub.2, 15B.sub.1 and 15B.sub.2, . . . are lined-up and installed in front of each one of a plurality of wire bonders 1A through 1D, and a guide device 3 is provided along the conveyors. As typically shown in the wire bonder 1C, each wire bonder includes a supply device 5 and a discharge device 8; and one of the two conveyors, which is 15C.sub.1, is installed to correspond to (or work with) the supply device 5, and the other conveyor, which is 15C.sub.2, is installed to correspond to (or work with) the discharge devices 8.
The conveyors 15A.sub.1, 15A.sub.2, 15B.sub.1, 15B.sub.2, . . . are respectively actuated by motors 16A.sub.1, 16A.sub.2, 16B.sub.2, 16B.sub.2 . . . In addition, sensors 17A.sub.1, 17A.sub.2, 17B.sub.1, 17B.sub.2, . . . which detect the passage of the workpieces are installed above or below the respective conveyors 15A.sub.1, 15A.sub.2, 15B.sub.1, 15B.sub.2, . . . Furthermore, stoppers 18A, 18B, 18C and 18D are installed near the conveyors 15A.sub.1, 15B.sub.1, 15C.sub.1 and 15D.sub.1 which are provided in front of the supply devices 5. The stoppers move up and down, and when they move up, they can stop the workpieces to position them on the conveyors.
A workpiece supply magazine 9 and a workpiece storage magazine 10 are installed on either end of the conveyor system.
When, for example, the wire bonder 1B requires a supply of a workpiece (or when the bonder 1B is actuated to perform the bonding), the conveyor 15B.sub.1 and all the conveyors between the conveyor 15B.sub.1 and the supply magazine 9 are actuated. In particular, the motors 16A.sub.1, 16A.sub.2 and 16B.sub.1 are actuated, and the conveyors 15A.sub.1, 15A.sub.2 and 15B.sub.1 in this case, are driven, and a workpiece is supplied to the conveyor 15A.sub.1 from the supply magazine 9 so that the workpiece is forwarded to the bonder 1B.
When the sensor 17A.sub.2 detects the workpiece, the stopper 18B is moved upward, and when the sensor 17B.sub.1 detects the conveyed workpiece, the conveyor 15B.sub.1 rotates for a fixed period of time and then stops. As a result, the workpiece sent onto the conveyor 15B.sub.1 is positioned by the stopper 18B in front of the supply device 5 of the bonder 1B and supplied into the wire bonder 1B by the supply device 5.
As seen from the above, when a workpiece is positioned by, for example, the stopper 18B, the motor 16B.sub.1 is actuated for a fixed period of time after the workpiece has been detected by the sensor 17B.sub.1, and then the conveyor 15B.sub.1 is stopped. In this case, since the speed of the conveyor 15B.sub.1 is high, the inertia of the workpiece causes the workpiece to hit the stopper 18B hard. If this happens, the workpiece, particularly its leading edge, may be damaged. Also, the workpieces may bounce back when they hit the stoppers. If this happens, the workpieces stop at different positions on the conveyors.